Zinc oxide films containing p-type dopant and process for preparing same

ABSTRACT

A p-type zinc oxide film and a process for preparing the film is disclosed. In a preferred embodiment, the p-type zinc oxide film contains arsenic and is grown on a gallium arsenide substrate. The p-type zinc oxide film has a net acceptor concentration of at least about 10 15  acceptors/cm 3 , a resistivity of no greater than about 1 ohm-cm, and a Hall mobility of between about 0.1 and about 50 cm 2 /Vs.

[0001] This invention was made with Government support underGrant/Project Number DAAH04-94-G-0305 awarded by the Army ResearchOffice. The Government may have certain rights in the invention.

BACKGROUND OF THE INVENTION

[0002] This invention is directed to zinc oxide (ZnO) films for use inelectrically excited devices such as light emitting devices (LEDs),laser diodes (LDs), field effect transistors (FETs), and photodetectors.More particularly, this invention is directed to ZnO films containing ap-type dopant for use in LEDs, LDs, FETs, and photodetectors whereinboth n-type and p-type materials are required, for use as a substratematerial for lattice matching to other materials in such devices, andfor use as a layer for attaching electrical leads.

[0003] For some time there has been interest in producing II-VI compoundwide band gap semiconductors to produce green/blue LEDs, LDs and otherelectrical devices. Historically, attempts to produce these devices havecentered around zinc selenide (ZnSe) or gallium nitride (GaN) basedtechnologies. However, these approaches have not been entirelysatisfactory due to the short lifetime of light emission that resultsfrom defects, and defect migration, in these devices.

[0004] Recently, because ZnO has a wide direct bandgap of 3.3 eV at roomtemperature and provides a strong emission source of ultraviolet light,ZnO thin films on suitable supporting substrates have been proposed asnew materials for light emitting devices and laser diodes. Undoped, aswell as doped ZnO films generally show n-type conduction. Impuritiessuch as aluminum and gallium in ZnO films have been studied by Hiramatsuet al. who report activity as n-type donors (Transparent Conduction ZincOxide Thin Films Prepared by XeCl Excimer Laser Ablation, J. Vac. Sci.Technol. A 16(2), Mar/Apr 1998). Although n-type ZnO films have beenavailable for some time, the growth of p-type ZnO films necessary tobuild many electrical devices requiring p-n junctions has to date beenmuch slower in developing.

[0005] Minegishi et al. (Growth of P-Type ZnO Films by Chemical VaporDeposition, Jpn. J. Appl. Phys. Vol. 36 Pt. 2, No. 11A (1997)) recentlyreported on the growth of nitrogen doped ZnO films by chemical vapordeposition and on the p-type conduction of ZnO films at roomtemperature. Minegishi et al. disclose the growth of p-type ZnO films ona sapphire substrate by the simultaneous addition of NH₃ in carrierhydrogen and excess Zn in source ZnO powder. When a Zn/ZnO ratio of 10mol % was used, secondary ion mass spectrometry (SIMS) confirmed theincorporation of nitrogen into the ZnO film, although the nitrogenconcentration was not precisely confirmed. Although the films preparedby Minegishi et al. using a Zn/ZnO ratio of 10 mol % appear toincorporate a small amount of nitrogen into the ZnO film and convert theconduction to p-type, the resistivity of these films is too high forapplication in devices such as LEDs or LDs. Also, Minegishi et al.report that the carrier density for the holes is 1.5×10¹⁶ holes/cm³,which is considered to be too low for use in commercial light emittingdevices or laser diodes.

[0006] Park et al. in U.S. Pat. No. 5,574,296 disclose a method ofproducing thin films on substrates by doping IIB-VIA semiconductors withgroup VA free radicals for use in electromagnetic radiation transducers.Specifically, Park et al. describe ZnSe epitaxial thin films doped withnitrogen or oxygen wherein ZnSe thin layers are grown on a GaAssubstrate by molecular beam epitaxy. The doping of nitrogen or oxygen isaccomplished through the use of free radical source which isincorporated into the molecular beam epitaxy system. Using nitrogen asthe p-type dopant, net acceptor densities up to 4.9×10¹⁷ acceptors/cm³and resistivities less than 15 ohm-cm were measured in the ZeSe film.However, the net acceptor density is too low and the resistivity is toohigh for use in commercial devices such as LEDs, LDs, and FETs.

[0007] Although some progress has recently been made in the fabricationof p-type doped ZnO films which can be utilized in the formation of p-njunctions, a need still exists in the industry for ZnO films whichcontain higher net acceptor concentrations and possess lower resistivityvalues.

SUMMARY OF THE INVENTION

[0008] Among the objects of the present invention, therefore, are theprovision of a ZnO film containing a high net acceptor concentration ona substrate; the provision of a process for producing ZnO filmscontaining p-type dopants; the provision of a process for producing p-njunctions utilizing a ZnO film containing a p-type dopant; the provisionof a process for producing homoepitaxial and heteroepitaxial p-njunctions utilizing a ZnO film containing a p-type dopant; and theprovision of a process for cleaning a substrate prior to growing a filmon the substrate.

[0009] Briefly, therefore, the present invention is directed to a ZnOfilm on a substrate wherein the film contains a p-type dopant. The filmhas a net acceptor concentration of at least about 10¹⁵ acceptors/cm³, aresistivity less than about 1 ohm-cm, and a Hall Mobility of betweenabout 0.1 and about 50 cm²/Vs.

[0010] The invention is further directed to a process for growing ap-type ZnO film containing arsenic on a GaAs substrate. The GaAssubstrate is first cleaned to ensure that the film will have a reducednumber of defects and will properly adhere to the substrate. Aftercleaning the temperature in the chamber is adjusted to between about300° C. and about 450° C. and the excimer pulsed laser is directed ontoa polycrystalline ZnO crystal to grow a film on the substrate. Thetemperature of the deposition chamber containing the substrate coatedwith the film is then increased to between about 450° C. and about 600°C. and the substrate is annealed for a time sufficient to diffusearsenic atoms into the film so as to produce a net acceptorconcentration of at least about 10¹⁸ acceptors/cm³ in the film.

[0011] The invention is further directed to a process for growing ap-type zinc oxide film on a substrate. The substrate is first cleaned toensure that the film will have a reduced number of defects and willproperly adhere to the substrate. After cleaning the substrate, thetemperature in the chamber is adjusted to between about 300° C. andabout 450° C., and a p-type zinc oxide film is grown on the substrate bydirecting an excimer pulsed laser beam onto a pressed ZnO powder pelletcontaining a p-type dopant to grow a p-type zinc oxide film containing anet acceptor concentration of at least about 10¹⁸ acceptors/cm³.

[0012] The invention is further directed to a process for preparing ap-n junction having a p-type ZnO film and an n-type film wherein the netacceptor concentration is at least about 10¹⁸ acceptors/cm⁻³. Asubstrate is loaded into a pulsed laser deposition chamber and cleanedto ensure that the film will have a reduced number of defects and willproperly adhere to the substrate. The temperature in the depositionchamber is then raised to between about 300° C. and about 450° C.Subsequently a p-type ZnO film having a net acceptor concentration of atleast about 10¹⁸ acceptors/cm³ is grown on the substrate by directing anexcimer laser onto a pressed ZnO powder pellet containing the p-typedopant. Finally an n-type film is grown on top of the p-type film bydirecting an excimer laser beam onto a pressed ZnO pellet containing then-type dopant.

[0013] The invention is further directed to a process for preparing ap-n junction having a p-type ZnO film and an n-type film wherein the netacceptor concentration is at least about 10¹⁸ acceptors/cm⁻³. Asubstrate is loaded into a pulsed laser deposition chamber and cleanedto ensure that the film will have a reduced number of defects and willproperly adhere to the substrate. The temperature in the depositionchamber is then raised to between about 300° C. and about 450° C.Subsequently an n-type film is grown on the substrate by directing anexcimer pulsed laser beam onto a pressed powder pellet containing ann-type dopant element. Finally, a p-type ZnO film is grown on the n-typefilm by directing an excimer pulsed laser beam onto a pressed ZnO powderpellet containing a p-type dopant element to a p-type ZnO film having anet acceptor concentration of at least about 10¹⁸ acceptors/cm³.

[0014] The invention is further directed to a process for cleaning asubstrate prior to growing a film on the substrate. A substrate isloaded into a chamber, the temperature is adjusted to between about 400°C. and about 500° C., and the chamber is filed with hydrogen to create apressure between about 0.5 and about 3 Torr. The distance between ametal shutter in the chamber and the substrate is adjusted to betweenabout 3 and about 6 centimeters and an excimer pulsed laser having anintensity between about 20 and about 70 mJ and a repetition of betweenabout 10 to about 30 Hz is directed onto the shutter for a period ofbetween about 5 and about 30 minutes to clean the substrate.

[0015] Other objects and features of this invention will be in partapparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a schematic diagram of a pulsed laser deposition system.

[0017]FIG. 2 is a photoluminescence spectra at 20° K of a ZnO film andan arsenic-doped ZnO film prepared in accordance with the presentinvention.

[0018]FIG. 3 is a Secondary Ion Mass Spectroscopy (SIMS) plot of anarsenic doped ZnO film prepared in accordance with the presentinvention.

[0019]FIG. 4 is an Atomic Force Microscopy image of a ZnSe film on aGaAs substrate wherein the substrate was cleaned using the cleaningprocess of the present invention.

[0020]FIG. 5 is an Atomic Force Microscopy image of a ZnSe film on aGaAs substrate wherein the substrate was cleaned using a thermalprocess.

[0021] Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] In accordance with the present invention, it has been discoveredthat ZnO films containing high levels of p-type dopants can be grown onsubstrates utilizing a pulsed laser deposition process alone or incombination with an annealing step. Surprisingly, the p-type dopantlevel achieved in the ZnO film is sufficient to allow the p-type film tobe used to form p-n junctions useful in electrical andelectroluminescent devices, for use as a substrate material for latticematching to materials in such devices, and for use as a desirable layerfor attaching electrical leads.

[0023] Referring now to FIG. 1, there is shown a schematic diagram of apulsed laser deposition system. Such a system is one method that can beutilized to grow ZnO films containing p-type dopants on suitablesubstrates. Other methods of growing ZnO films containing p-type dopantson substrates may include molecular beam epitaxy (MBE), MBE inconjunction with laser ablation, and chemical vapor deposition (CVD).Suitable p-type dopants for use in the present invention include Group 1(also know as IA, which includes elements such as Li, Na, K, Rb, andCa), Group 11 (also known as IB, which includes elements such as Cu, Ag,and Au), Group 5 (also known as VB, which includes elements such as V,Nb, and Ta), and Group 15 (also known as VA, which includes elementssuch as N, P, Sb, and Bi), with arsenic being preferred.

[0024] Again referring to FIG. 1, there is shown a focusing lens 8capable of directing an excimer laser beam 10 through laser window 6into pulsed laser deposition chamber 2. The beam 10 can be directed ontoeither metal shutter 4 or target 18 depending upon the desiredprocessing step. The beam 10 impinges on either metal shutter 4 orproduces an ablation plume of ZnO material from the target 18 and ontosubstrate 12. During the process of growing ZnO films, gas inlet tube 14allows gas 16 into the chamber 2.

[0025] Before growth of the ZnO film on the substrate, the substrateshould be cleaned in order to remove surface contaminants such as oxygenand carbon to minimize the number of defects in the film and to ensuremaximum adherence of the film to the substrate. Conventional substratecleaning techniques including wet chemical treatments, thermal cleaning,hydrogen atom plasma treatments, or any combination thereof can be usedto sufficiently clean the substrate surface. In addition, a pulsedexcimer laser, such as a pulsed argon fluoride excimer laser, can beused to clean the substrate in situ.

[0026] To clean the substrate using a pulsed excimer laser, thetemperature in the pulsed laser deposition chamber 2 is first adjustedto between about 400° C. and about 500° C., preferably to about 450° C.,and the chamber 2 is filled with a gas such as hydrogen to create apressure of between about 0.5 and about 3 Torr, preferably between about1 to about 2 Torr. Referring again to FIG. 1, a metal shutter 4, whichmay be made from iron, for example, is inserted between the target 18and the substrate 12 such that the substrate is positioned between about3 and about 6 centimeters, preferably about 4 centimeters, in front ofthe metal shutter 4. The focusing lens 8 is removed from the system, andan excimer laser beam 10, such as an argon fluoride excimer laser beamhaving an intensity of between about 20 and about 70 mJ, preferablyabout 50 mJ and a repetition rate of about 10 to about 30 Hz, preferablyabout 20 Hz, is directed into the chamber 2 and onto the metal shutter 4for a period of between about 5 and about 30 minutes.

[0027] During this period of illumination of the metal shutter, thelaser beam interacts with the metal shutter and creates excited hydrogenatoms, photoelectrons, and photons that effectively remove contaminantsfrom the substrate surface. Using the pulsed excimer laser, thesubstrate surface can be effectively cleaned at a much lower temperaturethan that required by conventional techniques. The pulsed excimer lasercleaning process can be effectively utilized to clean GaAs, GaN,sapphire, and other substrates prior to the deposition of ZnO, ZnSe, GaNand other films. For example, FIGS. 4 and 5 show Atomic Force Microscopy(AFM) images of the surface morphology of a ZnSe film on GaAssubstrates. In FIG. 4, the GaAs substrate was cleaned prior to thedeposition of the ZnSe film using the cleaning process described aboveand has only a root mean surface roughness of about 1.05 nanometers. InFIG. 5, the GaAs substrate was cleaned prior to the deposition of theZnSe film by a thermal treatment process at a substrate temperature ofabout 570° C. and has a root mean surface roughness of about 6.65nanometers. As indicated in FIGS. 4 and 5, the cleaning process of thepresent invention leaves a much improved uniform surface for subsequentfilm growth which greatly improves adherence of the film.

[0028] After the period of illumination is complete, the hydrogen gas ispumped out of chamber 2, and the chamber 2 temperature is adjusted tobetween about 250° C. and about 500° C., preferably to between about300° C. and about 450° C., to grow the ZnO film.

[0029] After the substrate has been cleaned and the temperature in thechamber adjusted, the focusing lens 8 is replaced, the metal shutter 4is removed and the target is pre-ablated with a pulsed excimer laserhaving an intensity of between about 20 and about 70 mJ, preferablyabout 50 mJ, and a repetition rate of between about 10 and about 30 Hz,preferably about 20 Hz for a period of about 10 minutes.

[0030] After the pre-ablation is complete, the chamber 2 is filled withoxygen to a pressure of between about 20 and about 40 mTorr, preferablyabout 35 mTorr. The laser beam 10 is directed through focusing lens 8and laser window 6 onto the target 18 to produce an ablation plume ofZnO material that is adsorbed onto substrate 12. The target 18 isbetween about 5 and about 10 centimeters, preferably about 7 centimetersfrom the substrate 12. Suitable targets for use in the present inventioninclude polycrystalline ZnO and ZnO powder pellets containing a dopant.Suitable substrates for use in the present invention include galliumarsenide, sapphire, and ZnO. The laser beam 10 can have an intensity ofabout 90 mJ and a repetition of about 20 Hz, for example. The laser beam10 is directed at the target 18 for a period of about 0.5 to about 4hours, preferably about 1 to about 2 hours to grow a ZnO film onsubstrate 12 between about 0.5 and about 3 micrometers thick.

[0031] In a particularly preferred embodiment of the present invention,the target 18 is polycrystalline ZnO, the substrate 12 is galliumarsenide, and the p-type dopant is arsenic. If the growth of the ZnOfilm on the gallium arsenide substrate as described above occurred at atemperature of at least about 400° C., no further processing steps arenecessary, and the ZnO layer will contain a net acceptor concentrationof at least about 10¹⁵ acceptors/cm³, preferably between about 10¹⁸ andabout 10²¹ acceptors/cm³ as arsenic atoms will migrate from the galliumarsenide substrate into the ZnO film during the film growth at atemperature of at least about 400° C. Additionally, the film will have aresistivity of no more than about 1 ohm-cm, preferably between about 1and about 10⁻⁴ ohm-cm, and a Hall mobility of between about 0.1 andabout 50 cm²/Vs.

[0032] If the growth of the ZnO film on the gallium arsenide substrateoccurs below about 400° C., a further processing step (annealing) isrequired to diffuse arsenic from the substrate into the ZnO film. Thisannealing step consists of adjusting the temperature in the chamber 2 tobetween about 450° C. and about 600° C., preferably to about 500° C.,and filling the chamber 2 with a gas such as nitrogen or oxygen at apressure between about 0.5 and about 4 Torr, preferably about 1 to about2 Torr. The gallium arsenide substrate is annealed for a period ofbetween about 10 and about 60 minutes, preferably about 20 to about 30minutes to produce a net acceptor concentration of at least about 10¹⁵acceptors/cm³, preferably between about 10¹⁸ acceptors/cm³ and about10²¹ acceptors/cm³ from the substrate 12 into the ZnO film.

[0033] Without being bound to a particular theory, in the one preferredembodiment when arsenic dopant from a GaAs substrate is caused todiffuse into a ZnO film, superior results are achieved due insubstantial part to the fact that the p-type dopant elemental source isthe substrate itself. The p-type dopant elemental source is therefore inintimate contact with the film, which facilitates diffusion moreefficiently and to a greater degree as compared to processes in whichthe substrate is not the dopant source. In this particular embodiment,therefore, having the dopant source be the substrate facilitatesachieving the improvements in net acceptor concentration, resistivity,and Hall mobility described herein. Also, the cleaning process asdescribed herein utilized with the preferred film growing and annealingprocesses cleans the GaAs surface extremely well to remove contaminantssuch as carbon and oxygen without damaging the crystal structure. Theclean, non-damaged surface allows the ZnO film to grow with improvedcrystal alignment and with a reduced number of defects. This cleaningprocess therefore further facilitates diffusion of arsenic moreefficiently and to a greater degree, which results in improvements instructural, optical and electrical properties.

[0034] Alternatively, ZnO films containing p-type dopants such asarsenic on a substrate can be prepared using pressed ZnO powder pelletscontaining a p-type dopant as the target in the pulsed laser depositionchamber. This process does not require migration of the dopant from thesubstrate into the film, and hence no annealing step is required.

[0035] A ZnO film is grown on a suitable substrate using the pulsedlaser deposition method described above except that the target is apressed ZnO powder pellet that contains a small amount of elementalp-type dopant. The amount of dopant, such as arsenic, required in thepowder pellet to achieve a net acceptor concentration level of at leastabout 10¹⁵ acceptors/cm³, preferably between about 10¹⁸ acceptors/cm³and about 10²¹ acceptors/cm³ is determined by measuring the amount ofdopant in the ZnO film and adjusting the dopant level in the powderedpellet until the net acceptor concentration of at least about 10¹⁵acceptors/cm³, preferably between about 10¹⁸ acceptors/cm³ and about10²¹ acceptors/cm³ is reached. For example, secondary ion massspectroscopy (SIMS) can be used to determine the amount of dopant in theZnO film. Additionally, Hall measurements in combination with electricalresistivity measurements can be used to determine whether the ZnO filmis p-type or n-type, the net concentration of p-type or n-type carriersin the ZnO film, to determine the Hall mobility of the carriers, and todetermine the electrical resistivity of the ZnO film. One skilled in theart will realize that the amount of dopant required in the powderedpellet may depend on numerous factors including operating conditions,distances from the target to the substrate, the size and shape of thechamber, as well as other variables during growth.

[0036] The concentration of p-type dopant may be varied within thep-type film by using more than one target and by selecting the targetsource during growth that yields the desired acceptor concentrations inthe ZnO film. Such variations may be desirable in order to preparesurfaces onto which electrical leads may be attached that have desirableelectrical properties.

[0037] Also in accordance with the present invention homoepitaxial andheteroepitaxial p-n junctions containing p-type doped ZnO films may beproduced on suitable substrates such as gallium arsenide, sapphire andZnO. To produce a homoepitaxial p-n junction, a ZnO layer is first grownon the substrate utilizing a pressed ZnO powder pellet containing ap-type dopant such as arsenic as described above to obtain a netacceptor concentration of at least about 10¹⁵ acceptors/cm³, preferablybetween about 10¹⁸ acceptors/cm³ and about 10²¹ acceptors/cm³. Theconcentration of p-type dopant may be varied across the p-type film byusing more than one target and by selecting the target source duringgrowth that yields the desired acceptor carrier concentration in the ZnOfilm. Such variations may be desirable in order to prepare surfaces ontowhich electrical leads may be attached that have desirable electricalproperties.

[0038] To complete the homoepitaxial p-n junction, an n-type ZnO film isgrown on top of the p-type ZnO film on top of the substrate. The n-typeZnO film is grown on top of the p-type ZnO film utilizing a pressed ZnOpowder pellet containing an n-type dopant such as aluminum as describedabove to yield an n-type film having a net donor concentration of atleast about 10¹⁵ donors/cm³, preferably between about 10¹⁸ donors/cm³and about 10²¹ donors/cm³. As with the p-type film, the concentrationlevel of the n-type carriers may be varied across the film by employingmore than one target.

[0039] A heteroepitaxial p-n junction can also be produced in accordancewith the present invention. To prepare a heteroepitaxial p-n junction, ap-type ZnO film is grown on a suitable substrate as described above anda film containing an n-type dopant is grown on top of the p-type ZnOfilm. The n-type film, however, is not comprised of ZnO in aheteroepitaxial p-n junction wherein a p-type ZnO film is utilized.Another source such as zinc selenide is used to produce the n-type film.

[0040] The use of heteroepitaxial structure p-n junctions prepared inaccordance with the present invention provides additional materials forp-n junction and device fabrication so as to achieve an expanded rangeof bandgap energies, increased optical tuning ranges, increased devicelifetimes, more desirable processing parameters and conditions, as wellas other advantages that will be recognized by one skilled in the art.

[0041] It will be recognized by one skilled in the art that, similar tothe preparing of ZnO films on a substrate, the preparation ofhomoepitaxial and heteroepitaxial p-n junctions can be accomplishedusing additional techniques in place of pulsed laser deposition. Othertechniques include MBE, MBE with laser ablation, CVD, and MOCVD. It willalso be recognized that devices having a more complex structure such asn-p-n junctions, p-n-p junctions, FETs, photodetectors, lattice matchinglayers, and layers on which electrical leads may be attached can easilybe fabricated using the above-described techniques and processes.

[0042] In accordance with the present invention, p-type ZnO material maybe used as substrate material to reduce or eliminate problems associatedwith lattice mismatch. P-type ZnO material that has a sufficiently highnet acceptor concentration and low electrical resistivity can be usedfor forming electrical contacts with desirable properties on devices.For example, a template p-type ZnO layer can be synthesized ontwo-compound semiconductor substrates such as GaAs. This template wouldprovide a transition layer for growing epitaxial GaN-based materialswith a density of defects that is lower than would occur in GaN filmsgrown directly on GaAs.

[0043] The present invention is illustrated by the following examplewhich is merely for the purpose of illustration and is not to beregarded as limiting the scope of the invention or manner in which itmay be practiced.

EXAMPLE

[0044] In this example a ZnO film was synthesized on a gallium arsenidesubstrate and the film/substrate was annealed to diffuse p-type arsenicdopant from the substrate into the film to produce a p-type ZnO film ona gallium arsenide substrate.

[0045] A gallium arsenide substrate having the shape of a thin wafer andbeing about 1 centimeter by about 1 centimeter by about 0.05 centimeterswas loaded into a pulsed laser deposition chamber, the temperature setat 450° C., and the chamber filled with high purity hydrogen to apressure of about 2 Torr. An iron shutter was inserted in front of thegallium arsenide substrate to create a separation distance of 4centimeters between the substrate and the shutter. An argon fluorideexcimer pulsed laser beam having an intensity of 50 mJ and a repetitionrate of 20 Hz was directed at the metal shutter through a laser windowand the shutter was illuminated for about 20 minutes to clean thesubstrate. Subsequently, the hydrogen was pumped out of the chamber, andthe substrate temperature was decreased to about 300° C.

[0046] After the substrate was cleaned, the metal shutter was removedand a focusing lens was inserted in front of the laser window to focusthe laser beam. The polycrystalline ZnO target was pre-ablated with theexcimer pulsed laser beam which was operating at an intensity of about50 mJ and having a repetition of about 20 Hz for a period of about 10minutes. High purity oxygen gas was then introduced into the chamber tocreate a pressure of about 35 mTorr.

[0047] The excimer pulsed laser beam, operating at an intensity of about90 mJ and a repetition of about 20 Hz, was then directed at thepolycrystalline ZnO for a period of about 2 hours to grow a ZnO filmhaving a thickness of about 1.0 micrometers on the substrate.

[0048] After the film growth, the pressure in the chamber was adjustedto about 2 Torr, and the temperature increased to about 500° C. Thefilm/substrate was annealed for about 30 minutes to diffuse arsenicatoms from the substrate into the ZnO film. The annealing created anarsenic doped p-type ZnO film on the gallium arsenide substrate.

[0049]FIG. 2 shows a photoluminescence spectra at 20° K of the ZnO film(solid line) and the arsenic-doped ZnO film (dots) prepared in thisExample. The pumping excitation is from a pulsed nitrogen laser with apower density of 128 kW/cm². The spectra shows that for the ZnO film thedonor-bound excitonic peaks located at about 3.36 eV (3698 angstroms)are dominant. However, the arsenic doped ZnO film of the present exampleshows that the acceptor-bound excitonic peak located at about 3.32 eV(3742 angstroms) is the strongest peak. This feature of acceptor-boundexcitonic peaks indicates that the acceptor density is greatly increasedwith arsenic doping, and the ZnO film becomes p-type.

[0050]FIG. 3 shows a Secondary Ion Mass Spectroscopy (SIMS) plot of thearsenic doped ZnO film prepared in this Example. The plot shows theconcentration in atoms/cm³ of arsenic as a function of depth from thesurface of the arsenic doped ZnO film. This plot shows that the arsenicconcentration is about 10¹⁸ atoms/cm³ to about 10²¹ atoms/cm³ throughoutthe film.

[0051] In view of the above, it will be seen that the several objects ofthe invention are achieved.

[0052] As various changes could be made in the above-described processfor preparing p-type ZnO films without departing from the scope of theinvention, it is intended that all matter contained in the abovedescription be interpreted as illustrative and not in a limiting sense.

What is claimed is:
 1. A ZnO film on a substrate, the ZnO film containing a p-type dopant and having a net acceptor concentration of at least about 15¹⁸ acceptors/cm³, a resistivity no greater than about 1 ohm-cm, and a Hall Mobility of between about 0.1 and about 50 cm²/Vs.
 2. The film as set forth in claim 1 wherein the net acceptor concentration is between about 10¹⁸ acceptors/cm³ and about 10²¹ acceptors/cm³, the resistivity is between about 1 ohm-cm and about 10⁻⁴ ohm-cm, and the Hall Mobility is between about 0.1 and about 50 cm²/Vs.
 3. The film as set forth in claim 1 wherein the p-type ZnO film has a thickness of between about 0.5 and about 3 micrometers.
 4. The film as set forth in claim 1 wherein the p-type dopant is arsenic.
 5. The film as set forth in claim 1 wherein the substrate is GaAs.
 6. The film as set forth in claim 1 wherein the p-type dopant is arsenic and the substrate is GaAs.
 7. The film as set forth in claim 1 wherein the p-type dopant is selected from Group 1, 11, 5, and 15 elements.
 8. The film as set forth in claim 1 wherein the film is incorporated into a p-n junction.
 9. The film as set forth in claim 1 wherein the film is incorporated into a field effect transistor.
 10. The film as set forth in claim 1 wherein the film is incorporated into a light emitting diode.
 11. The film as set forth in claim 1 wherein the film is incorporated into a laser diode.
 12. The film as set forth in claim 1 wherein the film is incorporated into a photodetector diode.
 13. The film as set forth in claim 1 wherein the film is incorporated into a device as a substrate material for lattice matching to materials in the device.
 14. The film as set forth in claim 2 wherein the p-type ZnO film has a thickness of between about 0.5 and about 3 micrometers.
 15. The film as set forth in claim 2 wherein the p-type dopant is arsenic.
 16. The film as set forth in claim 2 wherein the substrate is GaAs.
 17. The film as set forth in claim 2 wherein the p-type dopant is arsenic and the substrate is GaAs.
 18. The film as set forth in claim 2 wherein the p-type dopant is selected from Group 1, 11, 5, and 15 elements.
 19. The film as set forth in claim 2 wherein the film is incorporated into a p-n junction.
 20. The film as set forth in claim 2 wherein the film is incorporated into a field effect transistor.
 21. The film as set forth in claim 2 wherein the film is incorporated into a light emitting diode.
 22. The film as set forth in claim 2 wherein the film is incorporated into a laser diode.
 23. The film as set forth in claim 2 wherein the film is incorporated into a photodetector diode.
 24. The film as set forth in claim 2 wherein the film is incorporated into a device as a substrate material for lattice matching to materials in the device.
 25. A process for growing a p-type ZnO film containing arsenic on a GaAs substrate in a pulsed laser deposition chamber, the process comprising: cleaning the GaAs substrate; adjusting the temperature in the pulsed laser deposition chamber to between about 300° C. to about 450° C.; pre-ablating a polycrystalline ZnO crystal; directing an excimer pulsed laser beam onto the polycrystalline ZnO crystal to grow a film on the GaAs substrate; increasing the temperature of the pulsed laser deposition chamber to between about 450° C. and about 600° C.; and annealing the ZnO coated GaAs substrate to diffuse at least about 1×10¹⁸ acceptors/cm³ from the GaAs into the ZnO film to produce an arsenic doped ZnO film.
 26. The process as set forth in claim 25 wherein the ZnO film has a thickness of between about 0.5 and about 3 micrometers.
 27. The process as set forth in claim 25 wherein the arsenic doped ZnO film has a net acceptor concentration of between about 1×10¹⁸ acceptors/cm³ and about 1×10²¹ acceptors/cm³, a resistivity of between about 1 and about 1×10⁻⁴ ohm-cm, and a Hall Mobility of between about 0.1 and about 50 cm²/Vs.
 28. The process as set forth in claim 25 wherein the substrate is cleaned in the pulsed laser deposition chamber using a pulsed excimer laser.
 29. The process for growing a p-type ZnO film on a substrate, the process comprising: cleaning the substrate; adjusting the temperature in the pulsed laser deposition chamber to between about 300° C. and about 450° C.; and growing a p-type ZnO film on the substrate by directing an excimer pulsed laser beam onto a pressed ZnO powder pellet containing a p-type dopant to grow a p-type ZnO film containing at least about 10¹⁸ acceptors/cm³ on the substrate.
 30. The process as set forth in claim 29 wherein the p-type ZnO film has a thickness of between about 0.5 and about 3 micrometers.
 31. The process as set forth in claim 29 wherein the p-type dopant is selected from group 1, 11, 5, and 15 elements.
 32. The process as set forth in claim 29 wherein the p-type ZnO film has a net acceptor concentration of between about 10¹⁸ acceptors/cm³ and about 10²¹ acceptors/cm³, a resistivity no greater than about 1 ohm-cm, and a Hall Mobility of between about 0.1 and about 50 cm²/Vs.
 33. The process as set forth in claim 29 wherein the p-type dopant is arsenic.
 34. The process as set forth in claim 29 wherein the substrate is cleaned in the pulsed laser deposition chamber using a pulsed excimer laser.
 35. A process for preparing a p-n junction having a p-type ZnO film and an n-type film wherein the net acceptor concentration is at least about 10¹⁸ acceptors/cm³, the process comprising: cleaning a substrate; adjusting the temperature in the pulsed laser deposition chamber to between about 300° C. to about 450° C.; growing a p-type ZnO film on the substrate by directing an excimer pulsed laser beam onto a pressed ZnO powder pellet containing a p-type dopant element to grow a p-type ZnO film containing at least about 10¹⁸ acceptors/cm³ on the substrate; and growing an n-type film on top of the p-type ZnO film by directing an excimer pulsed laser beam onto a pressed powder pellet containing an n-type dopant element to grow an n-type film on the p-type ZnO film on the substrate.
 36. The process as set forth in claim 35 wherein the n-type film has a thickness of between about 0.5 and about 3 micrometers and the p-type film has a thickness of between about 0.5 and about 3 micrometers.
 37. The process as set forth in claim 35 wherein the p-type dopant element is arsenic and the n-type dopant element is aluminum.
 38. The process as set forth in claim 35 wherein the p-n junction is a homoepitaxial p-n junction wherein the p-type film consists of arsenic and ZnO and the n-type film consists of an n-type dopant element and ZnO.
 39. The process as set forth in claim 35 wherein the p-n junction is a heteroepitaxial p-n junction wherein the p-type film consists of arsenic and ZnO and the n-type film contains an n-type dopant and has an energy bandgap different than ZnO.
 40. The process as set forth in claim 35 wherein the substrate is cleaned in the pulsed laser deposition chamber using a pulsed excimer laser.
 41. A process for preparing a p-n junction having a p-type ZnO film and an n-type film wherein the net acceptor concentration is at least about 10¹⁸ acceptors/cm³, the process comprising: cleaning a substrate; adjusting the temperature in the pulsed laser deposition chamber to between about 300° C. to about 450° C.; growing an n-type film on top of the substrate by directing an excimer pulsed laser beam onto a pressed powder pellet containing an n-type dopant element to grow an n-type film on the substrate; growing a p-type ZnO film on the n-type film by directing an excimer pulsed laser beam onto a pressed ZnO powder pellet containing a p-type dopant element to grow a p-type ZnO film containing at least about 10¹⁸ acceptors/cm³ on the n-type film.
 42. The process as set forth in claim 41 wherein the n-type film has a thickness of between about 0.5 and about 3 micrometers and the p-type film has a thickness of between about 0.5 and about 3 micrometers.
 43. The process as set forth in claim 41 wherein the p-type dopant element is arsenic and the n-type dopant element is aluminum.
 44. The process as set forth in claim 41 wherein the p-n junction is a homoepitaxial p-n junction wherein the p-type film consists of arsenic and ZnO and the n-type film consists of an n-type dopant element and ZnO.
 45. The process as set forth in claim 41 wherein the p-n junction is a heteroepitaxial p-n junction wherein the p-type film consists of arsenic and ZnO and the n-type film contains an n-type dopant and has an energy bandgap different than ZnO.
 46. The process as set forth in claim 41 wherein the substrate is cleaned in the pulsed laser deposition chamber using a pulsed excimer laser.
 47. A process for cleaning a substrate in a chamber prior to growing a film on the substrate, the process comprising: loading the substrate into the chamber and adjusting the temperature in the chamber to between about 400° C. and about 500° C.; filling the chamber with hydrogen to create a pressure in the chamber of between about 0.5 and about 3 Torr; adjusting the distance between a metal shutter in the chamber and the substrate to between about 3 to about 6 centimeters; and directing an excimer pulsed laser beam having an intensity of between about 20 to about 70 mJ and a repetition of between about 10 to about 30 Hz into the chamber for a period of between about 5 and about 30 minutes to illuminate the metal shutter and clean the substrate.
 48. The process as set forth in claim 47 wherein the chamber temperature is about 450° C., the metal shutter is about 4 centimeters from the substrate, and an argon fluoride pulsed excimer laser having an intensity of about 50 mJ and a repetition of about 20 Hz illuminates the shutter for about 20 minutes to clean the substrate.
 49. A p-type film on a substrate wherein the film contains a p-type dopant which is an element which is the same as an element which is constituent of the substrate.
 50. The p-type film as set forth in claim 49 wherein the p-type film comprises ZnO, the substrate comprises GaAs, and the element which is the p-type dopant and a constituent of the substrate is arsenic. 